22 ASTRONOMY • JUNE 2022
The hope is that within a
decade or two, we will have seen
lots of planets at different stages
of formation. This will stand in as
a kind of time-lapse and we can
judge how well predictions of pre-
vailing hypotheses, like streaming
instability, match up to actual
exoplanets. This ambitious ven-
ture received a kickstart in 2021with the discovery of the youngest
planet ever observed: 2M0437b.
The discovery image, taken by the
Subaru Telescope on Mauna Kea
in Hawaii, shows a world still
glowing hot from energy released
during its formation, meaning
it just recently (astronomically
speaking) finished accreting. The
study, led by Eric Gaidos of the
University of Hawai’i, also fills
in our picture of how quicklyplanetary systems form, because
the star is only about 2.5 million
years old.Gathering dust
Every star grows up on its own
schedule. The protostar stage is
like a star’s volatile teen years.
When its accretion disk stabi-
lizes and material stops falling
into the core, it becomes a main
sequence star. There may still
be a debris disk and the planets
around might still be figuring out
where they orbit, but accretion
has largely stopped. That doesn’t
mean there won’t be any more
accretion in the star’s future,
though. Depending on its mass,
when fusion ceases, it will then
transition into either a white
dwarf, a neutron star, or a black
hole, all of which can form accre-
tion disks of their own.
The supply for this new disk
can come from a variety of
sources. Compact objects, like
white dwarfs and black holes,
may siphon gas from a compan-
ion star. A white dwarf may also
pull in material that it puffed off
in the earlier red giant phase.
And when black holes grow and
merge to become the supermas-
sive black holes (SMBHs) at thecenters of galaxies, they draw
material from the vast roaming
stars, clouds, and nebulae within
the galaxy itself.
As material from the disk falls
into the central object — whether
a star, planet, or singularity — it
releases energy in the form of
radiation. The disk itself also
radiates as it swirls around the
gravity well and heats up, with
different factors like viscosity,
friction, and speed making some
parts hotter than others. The
stronger the draw of the central
object, the more powerful the
radiation emitted, as gas can be
transformed into plasma. The
groundbreaking 2019 image of
the supermassive black hole at the
center of the galaxy M87 is not of
the hole itself, but of the black
hole’s shadow on the charged
plasma swirling around it.
A black hole gains mass from
everything it accretes over time.
But while we understand how
Sun-sized black holes form, we
don’t know how SMBHs got as
big as they are. For example,
the SMBH at the center of the
Whirlpool Galaxy (M51) in
Canes Venatici has a mass equiv-
alent to 1 million Suns. There is
no way for a single small, stellar-
mass black hole to accrete enough
material to grow this large at the
universe’s current age.
“It’s one of the biggest myster-
ies of black hole research,” says
Joanna Piotrowska, a graduate
student at Cambridge University.
The laws of physics limit how
quickly an object can accrete
matter, called the Eddington
limit. Above that limit, the radia-
tion from the accretion disk is so
intense, it blows material away
— preventing more accretion
from happening. “The mass
of [SMBHs] exceeds what is
expected from continuous accre-
tion at the Eddington limit over
the lifetime of our universe,” says
Piotrowska.
One proposed solution is that
SMBHs were big to start with.
Perhaps in the early universe,In 2014, the
ALMA radio
telescope revealed
distinctive gaps in
HL Tauri’s accretion
disk. They mark
regions where planets
are accreting and
sweeping up material.
ALMA (ESO/NAOJ/NRAO)THER
E^ IS^
NO^ WAY^ FO
R^ A^ S
INGLE
STEL
LAR-
MASSBLA
CK^ HOLE^ TO
ACCR
ETE^
ENOU
GH^ MATER
IAL^ TO^
GROW
THIS
LAR
GE^ ATTHE
UNIV
ERSE
’S^ CURREN
T^ AG
E.^